Detecting and measuring the point of contact between two or more objects has many applications. In many fluid dispensing systems, it is desirable to maintain a constant distance between the dispenser and the surface to ensure a uniform deposition of fluid. A particular example of this is in the fabrication of biological microarrays, where thousands of small spots (on the order of 50-250 microns in diameter) of biological molecules are placed at precise locations on a chemically treated surface. The more uniform the shape and size of the spots on a microarray, the more reliable results obtained from it in an experimental or diagnostic capacity. An example of a fluid dispenser for manufacturing microarrays is disclosed in U.S. Pat. No. 6,874,699.
Uniform dispensing can be a challenge when the target surfaces have imperfections or are not perfectly level. The larger the area dispensed on, the harder it is to maintain a constant distance between the dispenser and the surface.
An elementary contact sensing device comprises an object moved in relation to another, recording the distance of travel, and the contacting event, and then calculating the distance from the point of origin to the point of contact. The contact event may be indicated by closing an electric circuit where the contact and target objects are electrical conductors. Other methods of detection measure other electrical properties such as impedance, resistance, capacitance, or phase.
Certain materials called piezoelectrics will expand or contract when exposed to an electrical potential, and will also generate an electric potential when deformed. By applying an alternating current to a piezoelectric element, it can be made to vibrate. Impedance can be used as an electrical measure of the mechanical response of piezoelectric element to a specific frequency of alternating current. By measuring impedance as a function of frequency, the vibrational response of a piezoelectric element can be observed. The impedance spectrum generated by scanning frequencies at discrete intervals provides a unique fingerprint of the vibrational properties of a piezoelectric element that will change with alterations in the element's environment, such as materials to which it is attached or to which it comes into contact.
There have been several applications developed which take advantage of changes in impedance spectra to detect changes in material properties at remote sites. For example, U.S. Pat. No. 4,307,610 discloses a method for measuring crack propagation in materials undergoing alternating stress. An alternating load is applied to a pre-cracked specimen, and the change in frequency response is registered. The load is applied as a high frequency pulsator having an air gap between two poles, one mounted on the sample. The piezoelectric element functions to convert mechanical signals into electrical signals. U.S. Pat. No. 6,094,971 describes a scanning-probe microscope in which a resonating piezoelectric element drives a tuning fork oscillator which vibrates a cantilever at a position close to the surface of a sample. When the cantilever comes into tapping contact with the surface, there is a decrease in oscillation amplitude and change in the impedance measured from the piezoelectric element. In this application, piezo-induced vibration is calibrated to a single pre-determined frequency.
In U.S. Pat. No. 4,540,981, a vibratory device for detecting liquid levels utilizes a piezoelectric element to transmit vibrations to a metal rod suspended in a liquid reservoir. Vibration is established over a range of frequencies to ensure that the rod is vibrated at its resonance frequency. As the rod is immersed in fluid, its vibration at resonance will be damped, providing an indication of a liquid level in a reservoir. U.S. Pat. Nos. 4,864,856 and 6,781,287 describe other such liquid level sensing devices also incorporating a piezoelectric element.